In electrostatic drum printing on sheets, such as in facsimile machines, an electrostatic latent image on the sheets is developed into a visible image, usually by applying a liquid suspension of toner particles to the sheets. The developer typically must remain in contact with the sheets for about one second to obtain adequate image contrast. Attempts have been made to increase the rate of printing by increasing the speed of rotation of the drum. However, this sometimes results in a loss of image contrast or density.
It is desirable to support toning shoes for applying liquid toner to sheets in such a way that allows a toner applicator to scan a sheet in a helical stripe pattern. As the drum rotates, the toning shoe is moved laterally parallel to the drum axis with the toner applicator contacting the supported sheet so that developer flowing in the applicator is directly applied to the sheet. A helical stripe scanning pattern results. When the sheet is laid out, the stripe pattern consists of adjacent columns on the developed sheet forming an image. An advantage of such a scanning method is that fluid developer or toner is applied to a localized area during drum rotation, corresponding to a helical stripe or column. Thus, a printer head need only write an electrostatic latent image in a helical stripe pattern and the expensive full width printer heads are unnecessary.
However, in order to properly scan in a helical pattern, the shoe must be placed against the drum slightly askew so that fluid developer applied by the shoe follows the helical stripe. Also, the sheet must be placed on the drum askew so that when it is finally laid out, the image is straight relative to the edges of the sheet. Unfortunately, the toner applicator may not tend to seat properly against the sheet under these conditions. One side of the applicator may be closer to the sheet than an opposite side causing uneven development and excessive leakage of developer. Differences in applicator spacing from the sheet on the order of a mil can adversely affect image quality. Uneven applicator pressure may also cause image smearing, background marking, and actual damage to the electrographic paper surface. An uneven drum surface may also cause improper seating of the shoe.
It is an object of the present invention to provide a toner applicator assembly which can seat properly against a sheet on a drum during helical scanning operation.